Somatic mosaicism has been described in several genetic diseases including Primary Immunodeficiency Diseases (e.g. Wiskott-Aldrich Syndrome [WAS], X-linked Severe Combined Immunodeficiency), Fanconi Anemia, Epidermolysis Bullosa, and Tyrosinemia. Although somatic mosaicism (resulting either from a reversion to the original wild-type sequence or from a second-site revertant mutation) is observed in a high frequency of patients with WAS and is clearly an important aspect of disease, our understanding of this phenomenon is presently severely limited. Our recent identification of an unprecedented diversity of revertant genotypes in a WAS patient challenges the current notion that spontaneous reversions are rare events and that their occurrence is restricted to long-lived progenitors. Rather, our data are consistent with a model in which spontaneous WAS mutations originate with some frequency in somatic cells;these cells are then are acted upon by in vivo selection enriching those with partial or full restoration of WAS protein (WASp) function. We propose a novel perspective in which to view somatic mosaicism - in which the spectrum of somatic mutations in the vicinity of a germ-line WAS mutation essentially explores the landscape of possible WASp amino acid sequences, ultimately favoring those with sufficient fitness for in vivo selection. In this pilot study, we propose to first comprehensively examine the WAS DNA sequence space explored by revertant genotypes in this WAS patient. Second, we will examine the WASp fitness landscape explored by these revertant genotypes, focusing on several critical functions of WASp in T-lymphocytes. We propose that viewing WAS somatic mosaicism from an evolutionary biology perspective offers a conceptual framework for understanding the origination of revertant WAS mutations and the in vivo selection of cells bearing these mutations. Public Health Relevance: This exploratory study seeks to identify critical events in the development of somatic mosaicism in patients with inherited genetic disease - in particular, the Wiskott-Aldrich Syndrome.